Catalytic paraffin aromatization reactions are strongly endothermic, requiring substantial quantities of heat at relatively high temperature. For example, the aromatization of C.sub.2 -C.sub.8 paraffins over a zeolite catalyst having the structure of ZSM-5 requires heat input of about 278-556 kCal per kilogram of feed (500-1000 BTU per pound) at a reaction temperature of about 510.degree. to about 705.degree. C. (950.degree. to 1300.degree. F.). The problem of transferring heat to the fluid bed catalytic paraffin aromatization process has been an obstacle to its commercial development.
Preheating the feedstock to impart sufficient sensible heat for the endothermic aromatization reaction is unattractive for at least two reasons. First, the quantity of sensible heat which can practically be carried by the feedstock is limited, and is typically insufficient to supply the heat of reaction required for industrially feasible conversion rates. For this reason, the reaction is generally self-quenching when feed preheat is used alone. Second, the feed preheat temperatures required to impart sufficient sensible heat to the paraffinic feedstock typically thermally crack the feedstock to less valuable, and more difficult to aromatize, C.sub.4 - light aliphatic gases.
Preheating the catalyst to a temperature of around 870.degree. C. (1600.degree. F.) in conjunction with the requisite catalyst circulation rate can also effectively transfer the necessary heat for paraffin aromatization to the reaction zone. However, this entails elevated catalyst temperatures which markedly and undesirably accelerate catalyst deactivation.
Circulating a gas through a heat exchanger positioned in the fluidized bed presents problems as well. The high temperatures required in the aromatization reactor bed tend to limit the temperature differences available between the circulating gas and the reactor bed. Further, the heat transfer coefficient between the inner walls of the heat exchanger tubes and the circulating gas is relatively low. Thus, for a given rate of heat transfer, the comparatively low values for the log mean temperature difference and heat transfer coefficient must be offset by relatively large heat transfer areas.
Because the heat exchanger tubes must be positioned within the fluidized bed reactor, more or larger tubes require a larger reactor to maintain the desired catalyst volume. Given that the reactor as well as the exchanger tubes must be alloyed or otherwise designed to withstand the 510.degree.-705.degree. C. reaction temperatures, an increase in size corresponds to markedly higher material and fabrication costs.
In addition to transferring heat to the aromatization reaction zone, it would be desirable to maximize conversion of paraffins to valuable olefinic and aromatic product components. Thus, a process which achieves both effectively delivers thermal energy to the aromatization reaction zone while tailoring yield to favor more valuable products would be highly desirable.